Since the invention of sodium/sulfur batteries, a principal problem associated with their use has been the corrosive action of sulfur and polysulfide melts present during operation of such a battery. Containers for holding such corrosive materials in certain applications must have a long life. For example, if a sodium/sulfur battery is to be used in a load leveling application, the load leveling battery container is expected to have a minimum life of 10 years. Load leveling applications are ones in which electrical energy is stored in a battery during nonpeak load conditions and dissipated from the battery during peak load conditions.
While no specific search was carried out in the U.S. Patent and Trademark Office, we are aware of the following U.S. patents directed to container materials for molten polysulfide salts. These patents will be discussed briefly below, but we feel that they do not in any manner suggest the specific solution to this problem that we are proposing herein. As an additional matter, we are aware of U.S. Pat. No. 4,060,412, issued Nov. 29, 1977, for a "Method for Preparing a Fiber Reinforced Metal Matrix Using Microscopic Fibers", which discloses a composite material which we now disclose as useable as a conductor material for a sodium/sulfur battery. This patent will also be discussed hereinbelow.
U.S. Pat. No. 4,024,320 discloses a current collecting pole associated with an alkali metal/sulfur cell which comprises a first layer of an electronically conducting material which is resistant to corrosive action of sulfur and alkali metal polysulfides (e.g., carbon or graphite). The first layer defines a continuous surface in contact with the surface and alkali metal polysulfides. There is also a second layer of a higher electronically conducting material which is in electrical contact with the first layer over the surface of the latter remote from the sulfur and alkali metal polysulfides. Generally, this patent refers to a graphite or carbon tube with a plated metal outer layer or to a graphite or carbon tube with a plated metal layer inside the tube. The difference in whether the layer is on the outside or on the inside of the carbon tube depends on whether the alkali metal/sulfur cell is of the alkali metal core design or of a sulfur core design.
U.S. Pat. No. 4,052,535 is directed to a sodium/sulfur cell having a solid electrolyte and a cathode current collector with a porous conductive matrix, e.g., carbon or graphite felt. The porous conductive matrix is in the region between the electrolyte and the current collector. The matrix is formed of a plurality of discrete elements with electronically conductive material, e.g., graphite foil, between the elements extending across the region between the current collector and the electrolyte to increase the conductivity across that region.
U.S. Pat. No. 4,125,682 relates to a sodium/sulfur electric cell. The cell comprises a cathode tank containing sulfur, a solid electrolyte tube disposed in the tank and containing sodium. The cathode tube is lined with a continuous strip of felt or fabric which is made of graphite wound in a spiral.
U.S. Pat. No. 4,129,690 discloses a sodium/sulfur cell in which the cathode current collector in the sodium/polysulfide cathodic reactant comprises an impermeable tube, e.g., a carbon or graphite tube, which is inserted into the cathodic reactant and contains a solid metal core, e.g., an aluminum core, and a deformable electronic conductor, e.g., graphite felt, as a conducting interface between the impermeable tube and the core. In this structure the electronic contact between the aluminum rod and the graphite tube is made by graphite felt. Electronic contact in this structure is achieved mechanically, e.g., by graphite felt compression.
U.S. Pat. No. 4,290,192 discloses a method of making a portion of a sodium/sulfur battery which defines the volume for the cathodic reactant materials which are sulfur and sodium polysulfide materials. The container portion is defined by an outer metal casing with a graphite liner contained therein, the graphite liner having a coating on its internal diameter for sealing off the porosity thereof. The steel outer container and the graphite pipe are united by a method which ensures that at operating temperatures of the battery relatively low electrical resistant exists between the two materials because they are in intimate contact with one another.
While several U.S. patents have been mentioned above, there are a substantial number of other U.S. patents which propose answers to the problem of corrosive attack on conductors by sulfur/polysulfide melts. The patents are too numerous to mention and they in fact do not in any way propose the solution to this problem which is proposed in this specification.
This specification proposes that a composite material be used in making a current collector for a sodium/sulfur battery. The current collector so-made is electronically conductive and resistant to corrosive attack by molten sulfur and polysulfide melts under battery operating conditions. The composite material proposed for use in making the current collector material is a material which is disclosed in the aforementioned U.S. Pat. No. 4,060,412. The material taught in that patent is one in which microscopic fibers are mixed with a metal powder such as aluminum to provide a mixture with randomly oriented fibers having the metal particles adhered thereto. The mixture proposed in the patent is extruded at room temperature at least three times and is then placed in a die cavity and subjected to a first pressure at room temperature. While adding no additional pressure, the patent proposes that the die cavity be heated to bring the mixture to a temperature above the solidus of the metal powder, and the volume of the die cavity be decreased to at least the theoretical volume necessary to receive the mixture and ensure that the mixture includeds no voids. The die cavity is then cooled and the resultant billet ejected therefrom. There is no proposal contained in the patent that this material would find any particular utility in the formation of current collectors for a sodium/sulfur battery.
As is well known to those skilled in the art, the principal problem associated with sodium/sulfur batteries is the corrosiveness of the sulfur/polysulfide melt. In order for sodium/sulfur batteries to find use in applications such as load leveling by electric utilities, the battery must have a useful life of at least 10 years. However, it has been found that current collectors for containing a sulfur/polysulfide material, which must also act as electronically conductive members, generally cannot be protected to withstand the sulfur/polysulfide attack for such extended periods of time.
Generally, a sodium/sulfur battery operates in a temperature range of 300.degree.-400.degree. C. There are two basic battery designs currently being used. A first battery design is the so-called "sodium-core" design. The second battery design is the so-called "sulfur-core" design.
In the sodium-core design, the sodium is stored inside a sodium ion conducting ceramic electrolyte which is usually in a form of a closed end cylindrical tube. The sulfur/polysulfide melt is outside the electrolyte with a porous carbon matrix (for example, graphite felt) and is contained within a metal container. During discharge, sodium ions pass through the ceramic electrolyte and combine with sulfide ions on the other side of the electrolyte to form sodium polysulfide. The current within the sulfur electrode is carried by the carbon matrix and sodium polysulfide melt to the outer metal container which acts as the positive current collector for the cell.
In the sulfur-core design, the sulfur/sodium polysulfide melt is stored within the ceramic electrolyte and the sodium is stored outside the electrolyte. In this design, a metal current collector, usually in the form of a cylindrical rod in cylindrical cell designs, is placed inside the electrolyte in the polysulfide melt to act as a current collector. In both designs, the metal current collectors, whether a sulfur container in the sodium-core design or a metal current collector in the sulfur-core design, have to be formed from or be protected by electrically conductive materials that are corrosion resistant to sulfur/polysulfide melts. In addition, such materials have to be capable of withstanding thermal cycling between room temperature and 400.degree. C. without any significant impairment of either the electrical or protective capabilities thereof.